Vertical power devices having retrograded-doped transition regions and insulated trench-based electrodes therein

ABSTRACT

A power field effect transistor utilizes a retrograded-doped transition region to enhance forward on-state and reverse breakdown voltage characteristics. Highly doped shielding regions may also be provided that extend adjacent the transition region and contribute to depletion of the transition region during both forward on-state conduction and reverse blocking modes of operation. In a vertical embodiment, the transition region has a peak first conductivity type dopant concentration at a first depth relative to a surface on which gate electrodes are formed. A product of the peak dopant concentration and a width of the transition region at the first depth is preferably in a range between 1×10 12  cm −2  and 7×10 12  cm −2 .

FIELD OF THE INVENTION

The present invention relates to semiconductor switching devices, andmore particularly to switching devices for power switching and poweramplification applications and methods of forming same.

BACKGROUND OF THE INVENTION

Power MOSFETs have typically been developed for applications requiringpower switching and power amplification. For power switchingapplications, the commercially available devices are typically DMOSFETsand UMOSFETs. In these devices, one main objective is obtaining a lowspecific on-resistance to reduce power losses. In a power MOSFET, thegate electrode provides turn-on and turn-off control upon theapplication of an appropriate gate bias. For example, turn-on in anN-type enhancement MOSFET occurs when a conductive N-typeinversion-layer channel (also referred to as “channel region”) is formedin the P-type base region in response to the application of a positivegate bias. The inversion-layer channel electrically connects the N-typesource and drain regions and allows for majority carrier conductiontherebetween.

The power MOSFET's gate electrode is separated from the base region byan intervening insulating layer, typically silicon dioxide. Because thegate is insulated from the base region, little if any gate current isrequired to maintain the MOSFET in a conductive state or to switch theMOSFET from an on-state to an off-state or vice-versa. The gate currentis kept small during switching because the gate forms a capacitor withthe MOSFET's base region. Thus, only charging and discharging current(“displacement current”) is required during switching. Because of thehigh input impedance associated with the insulated-gate electrode,minimal current demands are placed on the gate and the gate drivecircuitry can be easily implemented. Moreover, because currentconduction in the MOSFET occurs through majority carder transportthrough an inversion-layer channel, the delay associated with therecombination and storage of excess minority carriers is not present.Accordingly, the switching speed of power MOSFETs can be made orders ofmagnitude faster than that of bipolar transistors. Unlike bipolartransistors, power MOSFETs can be designed to withstand high currentdensities and the application of high voltages for relatively longdurations, without encountering the destructive failure mechanism knownas “second breakdown”. Power MOSFETs can also be easily paralleled,because the forward voltage drop across power MOSFETs increases withincreasing temperature, thereby promoting an even current distributionin parallel connected devices.

DMOSFETs and UMOSFETs are more fully described in a textbook by B. J.Baliga entitled Power Semiconductor Devices, PWS Publishing Co. (ISBN0-534-94098-6) (1995), the disclosure of which is hereby incorporatedherein by reference. Chapter 7 of this textbook describes power MOSFETsat pages 335-425. Examples of silicon power MOSFETs includingaccumulation, inversion and extended trench FETs having trench gateelectrodes extending into the N+ drain region are also disclosed in anarticle by T. Syau, P. Venkatraman and B. J. Baliga, entitled Comparisonof Ultralow Specific On-Resistance UMOSFET Structures: The ACCUFET,EXTFET, INVFET, and Convention UMOSFETs, IEEE Transactions on ElectronDevices, Vol. 41, No. 5, May (1994). As described by Syau et al.,specific on-resistances in the range of 100-250 μΩcm² wereexperimentally demonstrated for devices capable of supporting a maximumof 25 volts. However, the performance of these devices was limited bythe fact that the forward blocking voltage must be supported across thegate oxide at the bottom of the trench. U.S. Pat. No. 4,680,853 to Lidowet al. also discloses a conventional power MOSFET that utilizes a highlydoped N+ region 130 between adjacent P-base regions in order to reduceon-state resistance. For example, FIG. 22 of Lidow et al. discloses ahigh conductivity region 130 having a constant lateral density and agradient from relatively high concentration to relatively lowconcentration beginning from the chip surface beneath the gate oxide andextending down into the body of the chip.

FIG. 1(d) from the aforementioned Syau et al. article discloses aconventional UMOSFET structure. In the blocking mode of operation, thisUMOSFET supports most of the forward blocking voltage across the N-typedrift layer, which must be doped at relatively low levels to obtain ahigh maximum blocking voltage capability, however low doping levelstypically increase the on-state series resistance. Based on thesecompeting design requirements of high blocking voltage and low on-stateresistance, a fundamental figure of merit for power devices has beenderived which relates specific on-resistance (R_(on,sp)) to the maximumblocking voltage (BV). As explained at page 373 of the aforementionedtextbook to B. J. Baliga, the ideal specific on-resistance for an N-typesilicon drift region is given by the following relation:

R _(on,sp)=5.93×10⁻⁹(BV)²⁵  (1)

Thus, for a device with 60 volt blocking capability, the ideal specificon-resistance is 170 μΩcm². However, because of the additionalresistance contribution from the channel, reported specificon-resistances for UMOSFETs are typically much higher. For example, aUMOSFET having a specific on-resistance of 730 μΩcm² is disclosed in anarticle by H. Chang, entitled Numerical and Experimental Comparison of60V Vertical Double-Diffused MOSFETs and MOSFETs With A Trench-GateStructure, Solid-State Electronics, Vol. 32, No. 3, pp. 247-251, (1989).However, in this device a lower-than-ideal uniform doping concentrationin the drift region was required to compensate for the highconcentration of field lines near the bottom corner of the trench whenblocking high forward voltages. U.S. Pat. Nos. 5,637,989 and 5,742,076and U.S. application Ser. No. 08/906,916, filed Aug. 6, 1997, thedisclosures of which are hereby incorporated herein by reference, alsodisclose popular power semiconductor devices having vertical currentcarrying capability.

In particular, U.S. Pat. No. 5,637,898 to Baliga discloses a preferredsilicon field effect transistor which is commonly referred to as agraded-doped (GD) UMOSFET. As illustrated by FIG. 3 from the '898patent, a unit cell 100 of an integrated power semiconductor devicefield effect transistor may have a width “W_(c)” of 1 μm and comprise ahighly doped drain layer 114 of first conductivity type (e.g., N+)substrate, a drift layer 112 of first conductivity type having alinearly graded doping concentration therein, a relatively thin baselayer 116 of second conductivity type (e.g., P-type) and a highly dopedsource layer 118 of first conductivity type (e.g., N+). The drift layer112 may be formed by epitaxially growing an N-type in-situ dopedmonocrystalline silicon layer having a thickness of 4 μm on an N-typedrain layer 114 having a thickness of 100 μm and a doping concentrationof greater than 1×10¹⁸ cm⁻³ (e.g. 1×10¹⁹ cm⁻³) therein. The drift layer112 also has a linearly graded doping concentration therein with amaximum concentration of 3×10¹⁷ cm⁻³ at the N+/N junction with the drainlayer 114, and a minimum concentration of 1×10¹⁶ cm⁻³ beginning at adistance 3 μm from the N+/N junction (i.e., at a depth of 1 μm) andcontinuing at a uniform level to the upper face. The base layer 116 maybe formed by implanting a P-type dopant such as boron into the driftlayer 112 at an energy of 100 kEV and at a dose level of 1×10¹⁴ cm⁻².The P-type dopant may then be diffused to a depth of 0.5 μm into thedrift layer 112. An N-type dopant such as arsenic may also be implantedat an energy of 50 kEV and at dose level of 1×10¹⁵ cm⁻². The N-type andP-type dopants can then be diffused simultaneously to a depth of 0.5 μmand 1.0 μm, respectively, to form a composite semiconductor substratecontaining the drain, drift, base and source layers.

A stripe-shaped trench having a pair of opposing sidewalls 120 a whichextend in a third dimension (not shown) and a bottom 120 b is thenformed in the substrate. For a unit cell 100 having a width W_(c) of 1μm, the trench is preferably formed to have a width “W_(t)” of 0.5 μm atthe end of processing. An insulated gate electrode, comprising a gateinsulating region 124 and an electrically conductive gate 126 (e.g.,polysilicon), is then formed in the trench. The portion of the gateinsulating region 124 extending adjacent the trench bottom 120 b and thedrift layer 112 may have a thickness “T₁” of about 2000 Å to inhibit theoccurrence of high electric fields at the bottom of the trench and toprovide a substantially the gate insulating region 124 extendingopposite the base layer 116 and the source layer 118 may have athickness “T₂” of about 500 Å to maintain the threshold voltage of thedevice at about 2-3 volts. Simulations of the unit cell 100 at a gatebias of 15 Volts confirm that a vertical silicon field effect transistorhaving a maximum blocking voltage capability of 60 Volts and a specificon-resistance (RP_(sp,on)) of 40 μΩcm², which is four (4) times smallerthan the ideal specific on-resistance of 170 μΩcm² for a 60 volt powerUMOSFET, can be achieved. Notwithstanding these excellentcharacteristics, the transistor of FIG. 3 of the '898 patent may sufferfrom a relatively low high-frequency figure-of-merit (HFOM) if theoverall gate-to-drain capacitance (C_(GD)) is too large. Improper edgetermination of the MOSFET may also prevent the maximum blocking voltagefrom being achieved. Additional UMOSFETs having graded drift regions andtrench-based source electrodes are also disclosed in U.S. Pat. No.5,998,833 to Baliga, the disclosure of which is hereby incorporatedherein by reference.

Power MOSFETs may also be used in power amplification applications(e.g., audio or rf). In these applications the linearity of the transfercharacteristic (e.g., I_(d) v. V_(g)) becomes very important in order tominimize signal distortion. Commercially available devices that are usedin these power amplification applications are typically the LDMOS andgallium arsenide MESFETs. However, as described below, power MOSFETsincluding LDMOS transistors, may have non-linear characteristics thatcan lead to signal distortion. The physics of current saturation inpower MOSFETs is described in a textbook by S. M. Sze entitled “Physicsof Semiconductor Devices, Section 8.2.2, pages 438-451 (1981). Asdescribed in this textbook, the MOSFET typically works in one of twomodes. At low drain voltages (when compared with the gate voltage), theMOSFET operates in a linear mode where the relationship between I_(d)and V_(g) is substantially linear. Here, the transconductance (g_(m)) isalso independent of V_(g):

g _(m)=(Z/L)u _(ns) C _(ox) V _(d)  (2)

where Z and L are the channel width and length, respectively, u_(ns) isthe channel mobility, C_(ox) is the specific capacitance of the gateoxide, and V_(d) is the drain voltage. However, once the drain voltageincreases and becomes comparable to the gate voltage (V_(g)), the MOSFEToperates in the saturation mode as a result of channel pinch-off. Whenthis occurs, the expression for transconductance can be expressed as:

g _(m)=(Z/L)u _(ns) C _(ox)(V _(g) −V _(th))  (3)

where V_(g) represents the gate voltage and V_(th) represents thethreshold voltage of the MOSFET. Thus, as illustrated by equation (3),during saturation operation, the transconductance increases withincreasing gate bias. This makes the relationship between the draincurrent (on the output side) and the gate voltage (on the input side)non-linear because the drain current increases as the square of the gatevoltage. This non-linearity can lead to signal distortion in poweramplifiers. In addition, once the voltage drop along the channel becomeslarge enough to produce a longitudinal electric field of more than about1×10⁴ V/cm while remaining below the gate voltage, the electrons in thechannel move with reduced differential mobility because of carriervelocity saturation.

Thus, notwithstanding attempts to develop power MOSFETs for powerswitching and power amplification applications, there continues to be aneed to develop power MOSFETs that can support high voltages and haveimproved electrical characteristics, including highly linear transfercharacteristics when supporting high voltages.

SUMMARY OF THE INVENTION

Vertical power devices according to embodiments of the present inventionutilize retrograded-doped transition regions to enhance forward on-stateand reverse breakdown voltage characteristics. Highly doped shieldingregions may also be provided that extend adjacent the transition regionsand contribute to depletion of the transition regions during bothforward on-state conduction and reverse blocking modes of operation.

A vertical power device (e.g., MOSFET) according to a first embodimentof the invention comprises a semiconductor substrate having first andsecond trenches and a drift region of first conductivity type (e.g.,N-type) therein that extends into a mesa defined by and between thefirst and second trenches. The drift region is preferably nonuniformlydoped and may have a retrograded doping profile relative to an uppersurface of the substrate in which the first and second trenches areformed. In particular, the substrate may comprise a highly doped drainregion of first conductivity type and a drift region that extendsbetween the drain region and the upper surface. The doping profile inthe drift region may decrease monotonically from a nonrectifyingjunction with the drain region to the upper surface of the substrate andan upper portion of the drift region may be uniformly doped at arelatively low level (e.g., 1×10¹⁶ cm⁻³). First and second insulatedelectrodes may also be provided in the first and second trenches. Thesefirst and second insulated electrodes may constitute trench-based sourceelectrodes in a three-terminal device.

First and second base regions of second conductivity type (e.g., P-type)are also provided in the mesa. These base regions preferably extendadjacent sidewalls of the first and second trenches, respectively. Firstand second highly doped source regions of first conductivity type arealso provided in the first and second base regions, respectively. Aninsulated gate electrode is provided that extends on the mesa. Theinsulated gate electrode is patterned so that the upper surfacepreferably defines an interface between the insulated gate electrode andthe first and second base regions. Inversion-layer channels are formedwithin the first and second base regions during forward on-stateconduction, by applying a gate bias of sufficient magnitude to theinsulated gate electrode.

A transition region of first conductivity type is also provided in themesa. This transition region preferably extends between the first andsecond base regions and extends to the interface with the insulated gateelectrode. The transition region may also form a non-rectifying junctionwith the drift region and has a vertically retrograded firstconductivity type doping profile relative to the upper surface. Thisdoping profile has a peak doping concentration at a first depth relativeto the upper surface, which may extend in a range from about 0.2 to 0.5microns relative to the upper surface. Between the first depth and theupper surface, the doping profile is preferably monotonically decreasingin a direction towards the upper surface. A magnitude of a portion of aslope of this monotonically decreasing profile is preferably greaterthat 3×10²¹ cm⁻⁴. The establishment of a “buried” peak at the firstdepth may be achieved by performing a single implant step at respectivedose and energy levels or by performing multiple implant steps atrespective dose levels and different energy levels. The peak dopantconcentration in the transition region is preferably greater than atleast about two (2) times the transition region dopant concentration atthe upper surface. More preferably, the peak dopant concentration in thetransition region is greater than about ten (10) times the transitionregion dopant concentration at the upper surface.

According to preferred aspects of power devices of the first embodiment,a product of the peak first conductivity type dopant concentration inthe transition region (at the first depth) and a width of the transitionregion at the first depth is in a range between 1×10¹² cm⁻² and 7×10¹²cm⁻² and, more preferably, in a range between about 3.5×10¹² cm⁻² andabout 6.5×10¹² cm⁻². Depending on unit cell design within an integratedmulti-celled device, the product of the peak first conductivity typedopant concentration in the transition region and a width of thenon-rectifying junction between the transition region and the driftregion may also be in a range between 1×10¹² cm⁻² and 7×10¹² cm⁻². Aproduct of the peak first conductivity type dopant concentration in thetransition region, a width of the transition region at the first depthand a width of the mesa may also be set at a level less than 2×10¹⁵cm⁻¹. To achieve sufficient charge coupling in the drift region mesa, aproduct of the drift region mesa width and quantity of firstconductivity type charge in a portion of the drift region mesa extendingbelow the transition region is preferably in a range between 2×10⁹ cm⁻¹and 2×10¹⁰ cm⁻¹.

According to further aspects of the first embodiment, enhanced forwardon-state and reverse blocking characteristics can be achieved byincluding highly doped shielding regions of second conductivity typethat extend in the mesa and on opposite sides of the transition region.In particular, a first shielding region of second conductivity type isprovided that extends between the first base region and the drift regionand is more highly doped than the first base region. Similarly, a secondshielding region of second conductivity type is provided that extendsbetween the second base region and the drift region and is more highlydoped than the second base region. To provide depletion during forwardon-state and reverse blocking modes of operation, the first and secondshielding regions form respective P-N rectifying junctions with thetransition region. High breakdown voltage capability may also beachieved by establishing a product of the peak first conductivity typedopant concentration in the transition region and a width between thefirst and second shielding regions in a range between 1×10¹² cm⁻² and7×10¹² cm⁻².

Integrated vertical power devices according to a second embodiment ofthe invention preferably comprise active unit cells that provide forwardon-state current and dummy cells that remove heat from the active cellsduring forward on-state conduction and support equivalent maximumreverse blocking voltages. According to the second embodiment, eachintegrated unit cell may comprise an active unit cell and one or moredummy unit cells. In addition to the first and second trenches, a thirdtrench may be provided in the semiconductor substrate. The first andsecond trenches define an active mesa, in which an active unit cell isprovided, and the second and third trenches define a dummy mesatherebetween in which a dummy unit cell is provided. A dummy base regionof second conductivity type is provided in the dummy mesa preferablyalong with a dummy shielding region. The dummy base and shieldingregions preferably extend across the dummy mesa and may be electricallyconnected to the first and second source regions within the active unitcell. In the event one or more dummy unit cells is provided, uniformreverse blocking voltage characteristics can be achieved by making thewidth of the mesa, in which the active unit cell is provided, equal to awidth of the respective dummy mesa in which each of the dummy unit cellsis provided. Alternatively, and in place of the third dummy base region,a field plate insulating layer may be provided on an upper surface ofthe dummy mesa and a third insulated electrode may be provided in thethird trench. The source electrode may extend on the field plateinsulating layer and is electrically connected to the first, second andthird insulated electrodes within the trenches. In the event a fieldplate insulating layer is provided on the dummy mesa instead of using adummy base region, the spacing between the first and second trenchesneed not necessarily equal the spacing between the second and thirdtrenches in order to support maximum blocking voltages.

Additional embodiments of the present invention also include methods offorming vertical power devices. These methods preferably includeimplanting transition region dopants of first conductivity type at afirst dose level and first energy level into a surface of asemiconductor substrate having a drift region of first conductivity typetherein that extends adjacent the surface. An insulated gate electrodemay then be formed on the surface. The insulated gate electrode ispreferably patterned so that it extends opposite the implantedtransition region dopants. Shielding region dopants of secondconductivity type are then implanted at a second dose level and secondenergy level into the surface. This implant step is preferably performedin a self-aligned manner with respect to the gate electrode, by usingthe gate electrode as an implant mask. Base region dopants of secondconductivity type are also implanted at a third dose level and thirdenergy level into the surface, using the gate electrode as an implantmask Accordingly, the base and shielding region dopants are self-alignedto each other.

A thermal treatment step is then performed to drive the implantedtransition, shielding and base region dopants into the substrate anddefine a transition region, first and second shielding regions onopposite sides of the transition region and first and second baseregions on opposite sides of the transition region. The transitionregion extends into the drift region and has a vertically retrogradedfirst conductivity type doping profile therein relative to the surface.This retrograded profile is achieved by establishing a buried peakdopant concentration sufficiently below the surface. The first andsecond shielding regions form respective P-N rectifying junctions withthe transition region and the first and second base regions also formrespective P-N rectifying junctions with the transition region. The doseand implant energies associated with the base and shielding regiondopants are also selected so that the shielding regions are more highlydoped relative to the base regions and extend deeper into the substrate.

According to a preferred aspect of this embodiment, the first dose andenergy levels and a duration of the thermal treatment step are ofsufficient magnitude that a product of a peak first conductivity typedopant concentration in the transition region and a width of thetransition region, as measured between the first and second shieldingregions, is in a range between 1×10¹² cm⁻² and 7×10¹² cm⁻². The firstand second energy levels may also be set to cause a depth of a peaksecond conductivity type dopant concentration in the shielding region tobe within 10% of a depth of a peak first conductivity type dopantconcentration in the transition region, when the depths of the peaks aremeasured relative to the surface.

The step of implanting shielding region dopants is also preferablypreceded by the step of forming trenches in the semiconductor substrateand lining the trenches with trench insulating layers. Conductiveregions are also formed on the trench insulating layers. These trenchrelated steps may be performed before the step of implanting thetransition region dopants. In this case, the transition region dopantsare preferably implanted into the conductive regions within the trenchesand into mesas that are defined by the trenches. According to stillfurther preferred aspects of this embodiment, steps are also performedto increase maximum on-state current density within the power device byimproving the configuration of the source contact. In particular, thesource contact is formed on a sidewall of the trenches by etching backthe trench insulating layers to expose the source, base and shieldingregions and then forming a source contact that ohmically contacts theconductive regions and also contacts the source, base and shieldingregions at the sidewall of each trench.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a vertical power device according toa first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a vertical power device according toa second embodiment of the present invention.

FIG. 3 is a cross-sectional view of a vertical power device according toa third embodiment of the present invention.

FIG. 4 is a cross-sectional view of a vertical power device according toa fourth embodiment of the present invention.

FIG. 5 is a cross-sectional view of a vertical power device according toa fifth embodiment of the present invention.

FIG. 6 is a cross-sectional view of a vertical power device according toa sixth embodiment of the present invention.

FIG. 7 is a cross-sectional view of a vertical power device according toa seventh embodiment of the present invention.

FIG. 8A is a graphical illustration of a preferred verticallyretrograded doping profile across the transition region of theembodiment of FIG. 1, obtained by performing multiple implants oftransition region dopants at respective different energies.

FIG. 8B is a graphical illustration of a preferred vertical dopingprofile across the source, base and shielding regions of the embodimentof FIG. 1.

FIGS. 9A-9K are cross-sectional views of intermediate structures thatillustrate preferred methods of forming the vertical power device ofFIG. 5.

FIG. 10 is a cross-sectional view of a vertical power device accordingto another embodiment of the present invention.

FIG. 11 is a cross-sectional view of a vertical power device thatincludes a dummy gate electrode electrically connected to a sourceelectrode, according to another embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. It will also be understood that when a layer is referred to asbeing “on” another layer or substrate, it can be directly on the otherlayer or substrate, or intervening layers may also be present. Moreover,the terms “first conductivity type” and “second conductivity type” referto opposite conductivity types such as N or P-type, however, eachembodiment described and illustrated herein includes its complementaryembodiment as well. Like numbers refer to like elements throughout.

Referring now to FIG. 1, an integrated vertical power device 10according to a first embodiment of the present invention includes aplurality of active vertical power device unit cells locatedside-by-side in a semiconductor substrate. As illustrated, the powerdevice 10 comprises a highly doped drain region 100 of firstconductivity type (shown as N+) and a drift region 102 of firstconductivity type that forms a non-rectifying junction with the drainregion 100. A drain electrode 136 is also provided in ohmic contact withthe drain region 100. The drain region 100 may have a thickness in arange between about 10 microns and about 500 microns. The drift region102 is preferably nonuniformly doped. In particular, the drift region102 preferably has a graded doping profile which decreases monotonicallyin a direction extending from the non-rectifying junction to a firstsurface 102 a of the drift region 102. This graded doping profile may bea linearly graded doping profile that decreases from a preferred maximumdrift region dopant concentration in a range between about 1×10¹⁷ andabout 2.5×10¹⁷ cm⁻³ to a minimum dopant concentration. Accordingly, ifthe drain region 100 is doped at a level of about 1×10¹⁹ cm⁻³ orgreater, then the non-rectifying junction will be an abruptnon-rectifying junction. An upper portion of the drift region 102 may beuniformly doped at a level of about 1×10¹⁶ cm⁻³ and the uniformly dopedupper portion of the drift region 102 may have a thickness in a rangebetween about 0.5 and about 1.0 μm.

A plurality of trenches 104 may be formed in the drift region 102. Iftrenches are provided, the trenches 104 are preferably formedside-by-side in the drift region 102 as parallel stripe-shaped trenches,however, other less preferred trench shapes (e.g., ring-shaped) may alsobe used. As described herein, regions will be defined as separateregions if they appear as such when viewed in transverse cross-section.Each pair of trenches preferably defines a drift region mesa 102 btherebetween, as illustrated. An electrically insulating layer 106 isalso provided on the sidewalls and bottoms of the trenches 104. The“trench” insulating layer 106 may have a thickness of about 3000 Å,however, the thickness may vary depending, among other things, on therating of the power device 10. The electrically insulating layer 106 maycomprise silicon dioxide or another conventional dielectric material.Each of the trenches 104 is preferably filled with a conductive region110 that is electrically insulated from the drift region 102 by arespective electrically insulating layer 106. The conductive regions 110may constitute trench-based electrodes that are electrically connectedtogether by a source electrode 138. This source contact/electrode 138may extend on the first surface 102 a of the drift region 102, asillustrated.

Upper uniformly doped portions of the drift region mesas 102 bpreferably comprise respective transition regions 130 of firstconductivity type. The transition regions 130 form respectivenon-rectifying junctions with the drift region 102 and, depending onthickness, may form respective non-rectifying junctions with theuniformly doped upper portions of the drift region 102 or the gradeddoped portions of the drift region 102. For example, the uniformly dopedupper portions of the drift region 102 may have a thickness of about 1.0μm relative to the first surface 102 a and the transition regions 130may have thicknesses of about 0.7 μm relative to the first surface 102a. Moreover, according to a preferred aspect of the present invention,each transition region 130 has a first conductivity type doping profiletherein that is vertically retrograded relative to the first surface 102a. In particular, a peak first conductivity type dopant concentration ata first depth in the transition region is at least two (2) times greaterthan a value of the retrograded first conductivity type doping profileat the first surface 102 a. More preferably, the peak first conductivitytype dopant concentration in the transition region is at least about ten(10) times greater than the value of the first conductivity type dopantconcentration at the first surface. According to another preferredaspect, a slope of at least a portion of the retrograded firstconductivity type doping profile is greater than about 3×10²¹ cm⁻⁴. Thedoping profile in the transition region 130 also includes a high-to-lowgraded profile in a direction extending downward from the peak to thenon-rectifying junction between the transition region 130 and the driftregion 102. A desired doping profile may be achieved by performing asingle transition region implant step at relatively high energy and doseor performing multiple implant steps. For example, as illustrated byFIG. 8A, a relatively wide peak in the transition region doping profilemay be achieved by performing three implant steps at respective energies(and same or similar dose levels) to achieve first, second and thirdimplant depths of about 0.15, 0.3 and 0.45 microns, using a dopanthaving a characteristic diffusion length of about 0.1 microns.

Gate electrodes 118 are provided on the first surface 102 a, asillustrated. These gate electrodes 118 may be stripe-shaped and mayextend parallel to the trench-based electrodes 110. As illustrated, thegate electrodes 118 preferably constitute insulated gate electrodes(e.g., MOS gate electrodes). The vertical power device 10 also compriseshighly doped shielding regions 128 of second conductivity type (shown asP+) that are formed at spaced locations in the drift region mesas 102 b.These shielding regions 128 are preferably self-aligned to the gateelectrodes 118. Each of the shielding regions 128 preferably forms a P-Nrectifying junction with a respective side of the transition region 130and with a respective drift region mesa 102 b (or tail of the transitionregion 130). According to a preferred aspect of the present invention,the peak second conductivity type dopant concentration in each shieldingregion 128 is formed at about the same depth (relative to the firstsurface 102 a) as the peak first conductivity type dopant concentrationin a respective transition region 130. Base regions 126 of secondconductivity type (shown as P) are also formed in respective driftregion mesas 102 b. Each base region 126 is preferably self-aligned to arespective gate electrode 118. Highly doped source regions 133 of firstconductivity type (shown as N+) are also formed in respective baseregions 126, as illustrated. The spacing along the first surface 102 abetween a source region 133 and a respective edge of the transitionregion 130 defines the channel length of the power device 10. Thesesource regions 133 ohmically contact the source electrode 138. Edgetermination may also be provided by extending the source electrode 138over peripheral drift region extensions 102 c and by electricallyisolating the source electrode 138 from the peripheral drift regionextensions 102 c by a field plate insulating region 125.

The combination within each drift region mesa 102 b of (i) a pair ofspaced-apart shielding regions 128 and (ii) a preferred transitionregion 130 that extends between the shielding regions 128 and has avertically retrograded doping profile, can enhance the breakdown voltagecharacteristics of each active unit cell in the multi-celled powerdevice 10. In particular, the shielding regions 128 can operate to“shield” the respective base regions 126 by significantly suppressingP-base reach-through effects when the power device 10 is blockingreverse voltages and causing reverse current to flow through theshielding regions 128 instead of the regions 126. This suppression ofP-base reach-through enables a reduction in the channel length of thedevice 10. Moreover, the preferred retrograded doping profile in thetransition region 130 enables complete or full depletion of thetransition region 130 when the power device 10 is blocking maximumreverse voltages and the drift region mesa 102 b is supporting thereverse voltage.

Full depletion of the transition region 130 may also occur duringforward on-state conduction. In particular, full depletion duringforward operation preferably occurs before the voltage In the channel(at the end adjacent the transition region 130) equals the gate voltageon the insulated gate electrode 118. As used herein, the reference tothe transition region being “fully depleted” should be interpreted tomean that the transition region is at least sufficiently depleted toprovide a JFET-style pinch-off of a forward on-state current path thatextends vertically through the transition region 130. To achieve fulldepletion, the relatively highly doped shielding regions 128 of secondconductivity (e.g., P+) are provided in close proximity and on oppositesides of the transition region 130. As the voltage in the channelincreases during forward on-state conduction, the transition region 130becomes more and more depleted until a JFET-style pinch-off occurswithin the transition region 130. This JFET-style pinch-off in thetransition region 130 can be designed to occur before the voltage at thedrain-side of the channel (V_(cd)) equals the gate voltage (i.e.,V_(cd)≦V_(gs)). For example, the MOSFET may be designed so that thetransition region 130 becomes fully depleted when 0.1≦V_(cd)≦30.5 Voltsand V_(gs)=4.0 Volts. Use of the preferred transition region 130 enablesthe field effect transistor within the power device 10 to operate in alinear mode of operation during forward on-state conduction while adrain region of the transistor simultaneously operates in a velocitysaturation mode of operation. Other power devices that exhibit similarmodes of operation are described in U.S. application Ser. No.09/602,414, filed Jun. 23, 2000, entitled “MOSFET Devices Having LinearTransfer Characteristics When Operating in Velocity Saturation Mode andMethods of Forming and Operating Same”, assigned to the presentassignee, the disclosure of which is hereby incorporated herein byreference.

Simulations of the device of FIG. 1 were also performed for a unit cellhaving a trench depth of 4.7 microns, a trench width of 1.1 microns anda mesa width of 1.9 microns. A sidewall oxide thickness of 3000 Å wasalso used. The drift region had a thickness of 6 microns and theuniformly doped upper portion of the drift region had a thickness of 0.5microns. The concentration of first conductivity type dopants in theuniformly doped A upper portion of the drift region was set at 1×10¹⁶cm⁻³ and the drain region had a phosphorus doping concentration of5×10¹⁹ cm⁻³. The gate oxide thickness was set at 250 Å and a total gatelength (across the mesa) of 0.9 microns was used. The widths of theshielding, base and source regions (relative to the sidewalls) were0.65, 0.65 and 0.45 microns, respectively, and the channel length was0.2 microns. The width of the transition region (at the depth of thepeak concentration in the transition region) was set at 0.6 microns. Thedepths of the source, base, shielding and transition regions and theirpeak dopant concentrations can be obtained from the following Table 1and FIGS. 8A-8B, where Peak N_(d) and Peak N_(a) are the peak donor andacceptor concentrations.

TABLE 1 Implant Energy Implant Dose Region (KeV) (cm⁻²) Dopant PeakN_(d,a) cm⁻³ N+ source 40-50 1-5 × 10¹⁵ P, As 1 × 10²⁰ P-base 40-50 1-5× 10¹³ B 2 × 10¹⁸(surface); 4 × 10¹⁷ (channel max) P+ shield 100 1-5 ×10¹⁴ B 5 × 10¹⁸ N-transition 200 1-10 × 10¹²  P 1.3 × 10¹⁷

Based on the above characteristics and including variations of the peakdopant concentration in the transition region (Peak_(TR)) and width ofthe transition region (W_(TR)), the following simulated breakdownvoltages of Tables 2 and 3 were obtained. Medici™ simulation software,distributed by Avant!™ Corporation, was used to perform the devicesimulations.

TABLE 2 W_(TR)(μm) (Peak_(TR))(cm⁻³) BV (Volts) Q(#/cm²) 0.5 0.4 × 10¹⁷80  0.2 × 10¹³ 0.5 0.7 × 10¹⁷ 80 0.35 × 10¹³ 0.5 1.2 × 10¹⁷ 79  0.6 ×10¹³ 0.5 1.3 × 10¹⁷ 78 0.65 × 10¹³ 0.5 1.4 × 10¹⁷ 62  0.7 × 10¹³ 0.5 1.6× 10¹⁷ 35  0.8 × 10¹³ 0.5 1.9 × 10¹⁷ 20 0.95 × 10¹³ 0.5 2.5 × 10¹⁷ 91.25 × 10¹³

TABLE 3 W_(TR)(μm) (Peak_(TR))(cm⁻³) BV (Volts) Q(#/cm²) 0.3 1.4 × 10¹⁷80 0.42 × 10¹³ 0.4 1.4 × 10¹⁷ 80 0.56 × 10¹³ 0.5 1.4 × 10¹⁷ 62  0.7 ×10¹³ 0.6 1.4 × 10¹⁷ 37 0.84 × 10¹³ 0.7 1.4 × 10¹⁷ 24 0.98 × 10¹³

As determined by the inventor herein and illustrated by the simulationresults of Tables 2 and 3, power devices having high breakdown voltagescan be provided by establishing a product of the peak first conductivitytype dopant concentration in the transition region (at the first depth)and a width of the transition region at the first depth in a preferredrange that is between about 1×10¹² cm⁻² and about 7×10¹² cm⁻² and, morepreferably, in a range between about 3.5×10¹² cm⁻² and about 6.5×10¹²cm⁻². This narrower more preferred range can result in devices havinghigh breakdown voltage and excellent on-state resistancecharacteristics. Depending on unit cell design within an integratedmulti-celled device, the product of the peak first conductivity typedopant concentration in the transition region and a width of thenon-rectifying junction between the transition region and the driftregion may also be in a range between about 1×10¹² cm⁻² and about 7×10¹²cm⁻². A product of the peak first conductivity type dopant concentrationin the transition region, a width of the transition region at the firstdepth and a width of the mesa may also be set at a level less than about2×10¹⁵ cm⁻¹. To achieve sufficient charge coupling in the drift regionmesa, a product of the drift region mesa width and quantity of firstconductivity type charge in a portion of the drift region mesa extendingbelow the transition region is preferably in a range between about 2×10⁹cm⁻¹ and about 2×10¹⁰ cm⁻¹.

Referring now to FIGS. 2-7, additional embodiments of power devicesaccording to the present invention include the multi-celled power device20 of FIG. 2. This device 20 is similar to the device 10 of FIG. 1,however, antiparallel diodes are provided by Schottky rectifyingcontacts that extend between the source electrode 138 and the driftregion extensions 102 c. The power device 30 of FIG. 3 is also similarto the power device 20 of FIG. 2, however, a plurality of dummy unitcells are provided in dummy drift region mesas 102 d. Dummy shieldingregions (shown as P+) and dummy base regions (shown as P) are alsoprovided in the dummy drift region mesas 102 d. As illustrated, thedummy base regions electrically contact the source electrode 138. Thedummy base regions and dummy shielding region can be formed at the sametime as the base and shielding regions within the active unit cells.Depending on the thermal ratings of a multi-celled power device, one ormore dummy unit cells may be provided to facilitate heat removal fromeach active unit cell.

The multi-celled power device 40 of FIG. 4 is similar to the device 30of FIG. 3, however, the dummy drift region mesas 102 d (which may notcontribute to forward on-state conduction, but preferably supportequivalent reverse breakdown voltages) are capacitively coupled througha field plate insulating layer 125 to the source electrode 138. Incontrast to the widths of the dummy drift region mesas 102 d in FIG. 3,which should be equal to the widths of the drift region mesas 102 b ofthe active unit cells, the widths of the dummy drift region mesas 102 din FIG. 4 need not be equal. The power device 50 of FIG. 5 is similar tothe device 20 of FIG. 2, however, the electrically insulating layers 106on the sidewalls of the trenches have been recessed to enable directsidewall contact between the source electrode 138 and the source, baseand shielding regions within the active unit cells. The establishment ofthis direct sidewall contact increases the active area of the device 50by reducing and preferably eliminating the requirement that the sourceregions be periodically interrupted in a third dimension (not shown) inorder to provide direct contacts to the base regions.

The power device 60 of FIG. 6 illustrates a relatively wide active driftregion mesa 102 b with a centrally located base region 126 a andshielding region 128 a. The transition region 130 a may have the samecharacteristics as described above with respect to the transitionregions 130 within the power devices 10-50 of FIGS. 1-5. The powerdevice 70 of FIG. 7 is similar to the device 60 of FIG. 6, however, thecentrally located base region 126 a and shielding region 128 a of FIG. 6have been separated by a centrally located trench 104. The power device10′ of FIG. 10 is similar to the power device 10 of FIG. 1, however, theinsulated gate electrode 118 on each active mesa 102 b has been replacedby a pair of shorter insulated gate electrodes 118 a and 118b. For amesa having a width of 2.6 microns, the gate electrodes 118 a and 118bmay have a length of 0.3 microns, for example. The use of a pair ofshorter gate electrodes instead of a single continuous gate electrodethat extends opposite the entire width of the transition region 130 canreduce the gate-to-drain capacitance C_(gd) of the device 10′ andincrease high frequency power gain. The source electrode 138 alsoextends into the space between the gate electrodes 118 a and 118b, asillustrated by FIG. 10. The portion of the source electrode 138 thatextends into the space between the gate electrodes 118 a and 118b mayhave a length of about 0.2 microns. The insulator that extends directlybetween the source electrode 138 and the transition region 130 may be agate oxide and may have a thickness in a range between about 100 Å andabout 1000 Å. The sidewall insulator that extends between the sidewallsof the gate electrodes 118 a and 118 b and the source electrode 138 mayalso have a thickness in a range between about 1000 Å and about 5000 Å,however, other sidewall insulator thicknesses may also be used.According to another aspect of this embodiment, the portion of thesource electrode 138 that extends into the space between the gateelectrodes 118 a and 118 b may be formed by patterning a conductivelayer (e.g., polysilicon) used to form the gate electrode 118 a and 118b. In particular, a third “dummy” gate electrode 118 c may be patternedthat extends opposite the transition region 130. An illustration of avertical power device 10″ that utilizes a dummy gate electrode 118 c isprovided by FIG. 11. The device 10″ of FIG. 11 may otherwise be similarto the device 10′ of FIG. 10. Electrical contact between this thirddummy gate electrode 118 c and the source electrode 138 may be madeusing conventional back-end processing techniques.

Preferred methods of forming the vertical power device of FIG. 5 with a65 Volt product rating will now be described. As illustrated by FIG. 9A,these methods may include the step of epitaxially growing a drift region202 of first conductivity type (shown as N) on a highly doped siliconsubstrate 200 (e.g., N+ substrate). This highly doped substrate 200 mayhave a first conductivity type doping concentration therein of greaterthan about 1×10¹⁹ cm⁻³ and may have an initial thickness T_(s) of about500 microns. The epitaxial growth step is preferably performed whilesimultaneously doping the drift region 202 with first conductivity typedopants in a graded manner. To achieve a 65 Volt product rating, avertical power device having an actual blocking voltage of 75 Volts maybe required. To achieve this blocking voltage, trenches having a depthin a range between about 4.5-5 microns will typically be required. Tosupport trenches with this depth, a graded doped drift region 202 havinga thickness T_(d) of about 6 microns may be required. Preferably, adrift region 202 having a thickness of 6 microns will include auniformly doped region at an upper surface thereof. This uniformly dopedregion may have a thickness in a range between about 0.5 and 1.0 micronsand may be doped at a uniform level of about 1×10¹⁶ cm⁻³. Thegrade-doped portion of the drift region 202 may have a thickness of5.0-5.5 microns and may be graded from a doping level of 1×10¹⁶ cm⁻⁻³ ata depth of 0.5 or 1.0 microns, for example, to a higher level of atleast about 5×10¹⁶ cm⁻³ at a depth of 6.0 microns. The drift region 202may form an abrupt non-rectifying junction with the substrate 200.

Conventional selective etching techniques may then be performed using afirst etching mask (not shown) to define a plurality of parallelstripe-shaped trenches 204 in the drift region 202. Trenches 204 havingother shapes may also be used. For example, each pair of adjacenttrenches 204 may represent opposing sides of a respective ring-shapedtrench. These trenches 204 may have a depth D_(t) of 5 microns, forexample. Adjacent trenches 204 define drift region mesas 202 btherebetween, with the width W_(m) of each mesa 202 b controlled by thespacing between the adjacent trenches 204. As illustrated by FIG. 9B, athin thermal oxide layer 206 may then be grown at a low temperature onthe sidewalls and bottoms of the trenches 204 and on an upper surface202 a of each of the mesas 202 b. For example, this thin oxide layer 206may be grown for a duration of 30 minutes at a temperature of 900° C. ina wet O₂ ambient. This thermal growth step may result in an oxide layer206 having a thickness of about 700 Å. This thin oxide layer 206 can beused to improve the interface between the sidewalls of the trenches 204and subsequently formed regions within the trenches 204, by removingetching related defects. The thermal budget associated with this thermaloxide growth step should be insufficient to significantly alter thegraded doping profile in the drift region 202, however, the dopingconcentration at the surface 202 a of each mesa 202 b may increase as aresult of dopant segregation. A thick conformal oxide layer 208 may thenbe deposited at a low temperature to produce an electrically insulatingspacer on the sidewalls and bottoms of the trenches 204. For a 65 Voltproduct rating, the total oxide thickness (thermal oxide plus depositedoxide) may be 3000 Å.

Referring now to FIG. 9C, a conformal polysilicon layer 210 may then bedeposited using a low temperature CVD process. The thickness of thislayer should be sufficient to fill the trenches 204. The polysiliconlayer 210 may be in-situ doped (e.g., with phosphorus) so that a lowsheet resistance of 10 ohms/square is achieved. As illustrated by FIG.9D, the deposited polysilicon layer 210 may then be etched back usingconventional etching techniques. The duration of this etching step maybe sufficiently long that the polysilicon regions 210 a within eachtrench 204 are planar with the upper surfaces 202 a of the mesas 202 b.This etch back step may be performed without an etching mask. Referringnow to FIG. 9E, another etching step may then be performed with a secondmask (not shown) in order to selectively remove the oxide over the mesas202 b, but preserve the oxide within field oxide regions (not shown)that may be located around a periphery of the drift region 202. Thissecond mask may comprise a photoresist layer that has been patterned todefine an etching window that is within a border of an outside trench(not shown) that surrounds an integrated power device containing aplurality of the illustrated power devices as unit cells.

As Illustrated by FIG. 9F, a thin pad oxide layer 212 is then grown as ascreening oxide over the exposed upper surfaces of the mesas 202 b. Thisthin pad oxide layer 212 may have a thickness of about 250 Å. This thinpad oxide layer 212 may be grown for a duration of 10 minutes at atemperature of 900° C. in a wet O₂ ambient. Transition region dopants214 of first conductivity type may then be implanted using a blanketimplant step. In particular, transition regions having verticallyretrograded doping profiles therein relative to the upper surface 202 amay be formed by implanting phosphorus dopants at an energy level of 200keV and at a preferred dose level of 5×10¹² cm⁻². This energy level of200 keV and dose level of 5×10¹² cm⁻² may result in an N-type transitionregion having a peak Implant depth (N_(PID)) of about 0.25-0.3 micronsand a peak dopant concentration of about 1.3×10¹⁷ cm⁻³. As illustratedon the left side of FIG. 8A, the N-type dopant concentration in theN-type transition region may be set to a value of less than about 2×10¹⁶cm⁻³ at the upper surface 202 a.

Referring now to FIG. 9G, the pad oxide layer 212 is then removed and inits place a gate oxide layer 216 having thickness of about 500 Å may beformed. This gate oxide layer 216 may be provided by performing athermal oxidation step in a wet O₂ ambient for a duration of 20 minutesand at a temperature of 900° C. A blanket polysilicon layer 218 is thendeposited and patterned using a photoresist mask layer 220 (third mask),to define a plurality of gate electrodes 218. A sequence of self-alignedimplant steps are then performed. In particular, highly dopedself-aligned shielding regions of second conductivity may be formed inthe transition region by implanting shielding region dopants 222 (e.g.,boron) at an energy level of 100 keV and at a dose level of 1×10¹⁴ cm⁻².After thermal treatment, these energy and dose levels may ultimatelyresult in a shielding region having a peak boron concentration of about5×10¹⁸ cm⁻³ at a depth of about 0.3 microns, assuming a characteristicdiffusion length of about 0.1 microns. These shielding region dopants222 are preferably implanted using both the gate electrodes 218 and themask layer 220 as an implant mask. Self-aligned base regions of secondconductivity type may also be formed in the shielding regions byimplanting base region dopants 224 (e.g., boron) at an energy level of50 keV and at a dose level of 3×10 cm⁻². The locations of peakconcentrations of the shielding region dopants 222 and base regiondopants 224 within the mesas 202 b, are represented by the referencecharacters “+”. The peak concentration of the shielding region dopantsmay equal 3×10¹⁸ cm⁻³, at a depth of 0.25-0.3 microns. This depthpreferably matches the depth of the peak of the transition regiondopants.

Referring now to FIG. 9H, the mask layer 220 may be removed and then adrive-in step may be performed at a temperature of about 1000° C. andfor a duration of about 60 minutes to define self-aligned base regions226 (shown as P), self-aligned shielding regions 228 (shown as P+) andthe transition regions 230 (shown as N). This drive-in step, whichcauses lateral and downward diffusion of the implanted base, shieldingand transition region dopants, may provide the highest thermal cycle inthe herein described method. If the uniform and graded doping profile inthe drift region is significantly altered during this step, then theinitial drift region doping profile may be adjusted to account for thethermal cycle associated with the drive-in step. As illustrated by FIG.9H, the implant energies and duration and temperature of the drive-instep may be chosen so that the depth of the P-N junction between the P+shielding region 228 and the drift region 202 is about equal to thedepth of the non-rectifying junction between the transition region 230and the drift region 202, however, unequal depths may also be used. Thedepth of the P-N junction may equal 0.7 microns.

Referring now to FIG. 9I, source region dopants 232 of firstconductivity type are then implanted into the base regions 226, usingthe gate electrodes 218 as an implant mask. The source region dopants232 may be implanted at an energy level of 40 keV and at a dose level of2×10¹⁴ cm⁻². As illustrated by FIG. 9J, the implanted source regiondopants (shown by reference character “−”) may then be driven-in at atemperature of 900° C. and for a duration of 10 minutes, to define N+source regions 233. This implant step may be performed using the gateelectrodes 218 and fourth photoresist mask (not shown) as an implantmask. The fourth photoresist mask may be patterned to define thelocations of shorts to the P-base region in a third dimension relativeto the illustrated cross-section (not shown). Conventional insulatordeposition, sidewall spacer formation and patterning steps may then beperformed to define a plurality of insulated gate electrodes 234. Thesesteps may also be performed to define contact windows to the sourceregions, the P-base regions, the polysilicon in the trenches and thegate electrodes. The insulating regions 206/208 lining upper sidewallsof the trenches may also be selectively etched back to expose sidewallsof the source, base and shielding regions. The presence of this etchback step may eliminate the need to define shorts to the P-base region,using the fourth photoresist mask, and therefore may result in anincrease in the forward on-state conduction area for a given lateralunit cell dimension. As illustrated by FIG. 9K, conventional front sidemetallization deposition and patterning steps may also be performed todefine a source contact 238 and gate contact (not shown). Asillustrated, the source contact 238 extends along the upper sidewalls ofthe trenches 204 and contacts the exposed portions of the source, baseand shielding regions. The backside of the substrate 200 may also bethinned and then conventional backside metallization steps may beperformed to define a drain contact 236.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

What is claimed is:
 1. A vertical power device, comprising: asemiconductor substrate having first and second trenches and a driftregion of first conductivity type therein that extends into a mesadefined by the first and second trenches; first and second insulatedelectrodes in the first and second trenches; first and second baseregions of second conductivity type that extend adjacent sidewalls ofthe first and second trenches, respectively, and in the mesa; first andsecond source regions of first conductivity type in said first andsecond base regions, respectively; an insulated gate electrode thatextends on a surface of said semiconductor substrate and opposite saidfirst base region; and a transition region of first conductivity typethat extends between said first and second base regions, forms anon-rectifying junction with the drift region and has a verticallyretrograded first conductivity type doping profile relative to thesurface.
 2. The device of claim 1, wherein a peak first conductivitytype dopant concentration in said transition region is at least tentimes greater than a value of the retrograded first conductivity typedoping profile at the surface.
 3. The device of claim 1, wherein saidfirst and second insulated electrodes are electrically connected to saidfirst and second source regions.
 4. The device of claim 3, wherein saidfirst insulated electrode ohmically contacts said first base region andsaid first source region at the sidewall of the first trench.
 5. Thedevice of claim 1, wherein said transition region has a peak firstconductivity type dopant concentration therein at a first depth relativeto the surface; and wherein a product of the peak first conductivitytype dopant concentration in said transition region and a width of saidtransition region at the first depth is in a range between 1×10¹² cm⁻²and 7×10¹² cm⁻².
 6. The device of claim 5, wherein the peak firstconductivity type dopant concentration in said transition region isgreater than about 1×10¹⁷ cm⁻³; wherein the surface defines an interfacebetween said insulated gate electrode and said transition region; andwherein a first conductivity type dopant concentration in saidtransition region is less than about 2×10¹⁶ cm⁻³ at the surface.
 7. Thedevice of claim 1, wherein said transition region has a peak firstconductivity type dopant concentration therein at a first depth relativeto the surface; and wherein a product of the peak first conductivitytype dopant concentration in said transition region and a width of saidtransition region at the first depth is in a range between 3.5×10¹² cm⁻²and 6.5×10¹² cm⁻².
 8. The device of claim 1, wherein said transitionregion has a peak first conductivity type dopant concentration thereinat a first depth relative to the surface; and wherein a product of thepeak first conductivity type dopant concentration in said transitionregion and a width of the non-rectifying junction is in a range between1×10₁₂ cm⁻² and 7×10¹² cm⁻².
 9. The device of claim 1, wherein saidtransition region has a peak first conductivity type dopantconcentration therein at a first depth relative to the surface; andwherein a product of the peak first conductivity type dopantconcentration in said transition region, a width of said transitionregion at the first depth and a width of the mesa is less than 2×10¹⁵cm⁻¹.
 10. The device of claim 1, wherein said first source region andsaid first base region are self-aligned to said insulated gateelectrode.
 11. The device of claim 1, further comprising: a firstshielding region of second conductivity type that extends between saidfirst base region and the drift region and is more highly doped thansaid first base region; and a second shielding region of secondconductivity type that extends between said second base region and thedrift region and is more highly doped than said second base region. 12.The device of claim 11, wherein said first and second shielding regionsform respective P-N rectifying junctions with said transition region;wherein said transition region has a peak first conductivity type dopantconcentration therein at a first depth relative to the surface; andwherein a product of the peak first conductivity type dopantconcentration in said transition region and a width between said firstand second shielding regions is in a range between 1×10¹² cm⁻² and7×10¹² c⁻².
 13. The device of claim 11, wherein the non-rectifyingjunction extends between said first and second shielding regions. 14.The device of claim 1, wherein the drift region has a verticallyretrograded first conductivity type doping profile therein relative tothe surface.
 15. The device of claim 1, wherein said insulated gateelectrode extends opposite said first and second source regions, saidfirst and second base regions and said transition region.
 16. The deviceof claim 1, further comprising a third trench in said semiconductorsubstrate, said second and third trenches defining a dummy mesatherebetween into which the drift region extends.
 17. The device ofclaim 16, further comprising a third base region of second conductivitytype that extends in the dummy mesa and is electrically connected tosaid first and second source regions.
 18. The device of claim 17,wherein a width of the dummy mesa extending between said second andthird trenches equals a with of the mesa extending between said firstand second trenches.
 19. The device of claim 16, further comprising: athird insulated electrode in said third trench; a field plate insulatinglayer on the dummy mesa; and a source electrode that extends on saidfield plate insulating layer and is electrically connected to saidfirst, second and third insulated electrodes.
 20. The device of claim19, wherein a spacing between said first and second trenches is unequalto the spacing between said second and third trenches.
 21. A verticalpower device, comprising: a semiconductor substrate having first andsecond trenches and a drift region of first conductivity type thereinthat extends into a mesa defined by the first and second trenches; firstand second insulated electrodes in the first and second trenches; firstand second base regions of second conductivity type that extend adjacentsidewalls of the first and second trenches, respectively, and in themesa; first and second source regions of first conductivity type in saidfirst and second base regions, respectively; an insulated gate electrodethat extends on a surface of said semiconductor substrate and oppositesaid first base region; and a transition region of first conductivitytype that extends between said first and second base regions and forms anon-rectifying junction with the drift region, said transition regionhaving a peak first conductivity type dopant concentration therein at afirst depth relative to a surface of said substrate; and wherein aproduct of the peak first conductivity type dopant concentration in saidtransition region and a width of said transition region at the firstdepth is in a range between 3.5×10¹² cm⁻² and 6.5×10¹² cm⁻².
 22. Thevertical power device of claim 21, wherein a product of a width of themesa and a quantity of first conductivity type charge in a portion ofthe mesa extending below the transition region is preferably in a rangebetween 2×10⁹ cm⁻¹ and 2×10¹⁰ cm⁻¹.
 23. The vertical power device ofclaim 21, further comprising third and fourth trenches in saidsemiconductor substrate, said second and third trenches defining a firstdummy mesa therebetween into which the drift region extends and saidthird and fourth trenches defining a second dummy mesa therebetween intowhich th drift region extends.